Support cargo-apk on Android devices proper, e.g. Chromebook

[PaulAtBanno]

Right now, cargo-apk seems to only support building on Linux, macOS, or Windows; attempting to use under Android itself results in an "Unknown or incompatible host" error. This appears to be due to a case-by-case "target os," i.e. host os as far as the Rust compiler is concerned, test in build.rs. It may be sufficient just to add a check for android that does no transformation.

Alternatively, I suggest please looking into what would be involved in supporting Termux under Android in its own right, but that probably involves more work to understand how Termux lays out, e.g. the header and library locations of the relevant materials from the NDK, etc.